One of the major difficulties in reception of small signals is the presence of a large unwanted signal in the same frequency band.
Typical spectrometers sweep a narrowband window across the band of interest, thereby converting the wideband into a series of narrow bands. Within each narrowband window, the signal is characterized, and when the sweep is complete, the signal is logically reconstructed. By analyzing a narrowband component, the signal can be frequency translated or downconverted to a nominal one, permitting a static detector to be used. Frequency translation typically requires the mixing of the signal with a tuning frequency in an analog non-linear element, such as a semiconductor. The nonlinear mixer generates sum and difference frequencies, one of which (typically the difference) is selected for analysis. All nonlinear components in the analog signal processing chain will generate spurious components, including intermodulation (sum and difference components of the existing signals) as well as harmonics. In a high quality spectrometer, this spectral pollution is undesirable, especially where there are many possible signal interactions, or where the signal source to be characterized may itself produce harmonics and other components to be characterized. Even in a narrowband spectral analyzer, nonlinear distortion may be a problem, since out of narrowband components may be translated into the band of interest. Another issue for swept narrowband spectrometers is that they cannot analyze a spectrum in realtime or near realtime, since there is a limited speed of sweep.
Another paradigm for constructing a wideband spectrometer is to translate the entire band of interest to baseband (i.e., a band from f1 to f2 is frequency translated to a band from 0 to (f2−f1) using a nonlinear mixer with a frequency f1) and digitized above the Nyquist rate (2 times the highest frequency component, i.e., 2(f2−f1)). The digitized signal is then processed using a digital signal processor, for example using a fast Fourier transform, to reveal the spectral energy. This approach has a number of limitations. As in the narrowband approach, a nonlinear analog mixer is employed, and thus spectral pollution occurs. One known system, Pinckley, U.S. Pat. No. 5,519,890, expressly incorporated herein by reference, downconverts a cellular band signal, and then employs a tunable filter bank to remove carriers having interfering signals before digitizing the band, which is then digitally demodulated by a bank of digital signal processors.
The third possibility, directly digitizing the signal of interest, without frequency translation, has been infeasible at high frequencies, e.g., above 500 MHz, or at high dynamic range-frequency products, at least because of digitizer limitations. One particular limitation, discussed above, is the large amplitude interferer problem. Since the digitizer operates across the entire spectrum, it can be saturated or dominated by a single signal, thus masking other components. As the bandwidth to be analyzed (or dynamic range-frequency product) is increased, the likelihood of the presence of interferers increases. One approach to solving this problem is to increase the dynamic range of the digitizer to an extent necessary to handle both the interferor and the signal of interest. However, this may be difficult, and the problem grows exponentially with each added bit of dynamic range required.
In military communications, the large signal interferor problem may occur due to transmitters co-located on the reception platform and is often referred to as the co-site interference problem. One method of dealing with this problem is to have a large dynamic range receiver, capable of simultaneously receiving the large interferor and the small signal of interest (SOI). In fact, it is not the dynamic range, but the instantaneous spur-free dynamic range of the receiver that needs to be large, often imposing a linearity-requirement that is impossible to meet, even with ultra-linear superconductor front-ends. This requirement could be relaxed substantially if one or more of these interferors could be excised from the incoming waveform through sharp notch filters. However, these interferors often shift in frequency, especially in the case of narrow-band high-power jammers. Therefore, a tunable notch filter would be needed, along with appropriate logic and control for tuning it.
While interference from intentional enemy jammers remains a major problem for receivers, the problem of co-site interference is particularly severe for maritime communications and surveillance systems. On a ship, multiple high power communications and radar transmitters exist in close proximity to RF receivers on the same platform. Consequently, the receive antenna picks up a part of these transmit signals. Since military tactical communication systems are rapidly migrating towards wide bandwidths (hundreds of MHz to a few GHz), supporting multiple narrowband and broadband waveforms, the number of interferers in the wide receive band continue to rise. Co-site interference manifests itself in three forms:                1) Small signal of interest in the presence of large interfering signal (FIG. 2A),        2) Small signal of interest in the presence of a large number of signals of comparable power (FIG. 2B),        3) Impulsive interference from hoppers.        
The worst problem occurs through a large in-band interferer that drives the receiver into saturation. This creates non-linear distortions or spurs, preventing detection of the much smaller signal-of-interest. Spurs also occur from in-band intermodulation products from large out-of-band interferers. The presence of these spurious signals and other small interferors prevents full usage of the receiver spectrum. Finally, the transients from hopping transmitters also cause interference. All these effects severely limit functionality of RF receivers, and indirectly the co-located transmitters. The following difficulties arise from the inability of current communication systems to reject, cancel, or tolerate co-site interference:
DifficultiesConsequencePoor spectrum efficiency and wastage ofInformation capacity is compromisedavailable spectral resourceThe number of hoppers than can be supportedFewer channels of secure communicationson a platform is limitedare availableSmall signals of interest cannot be detected;Shorter communication range; Increasedlow probability of intercept signaling affectedvulnerability to interception of communi-cation by enemyDynamic frequency and bandwidth allocationLonger operational planning time and schemes are not permittedreduced agility in battle situations
The situation is even worse for surveillance (e.g. SIGINT) receivers. These very wideband receivers, attempting to listen for weak signals, can be rendered useless by large co-site interferers. Often one has to resort to the extremely undesirable solution of shutting down the SIGINT receiver for short periods of time to combat the self-jamming from co-located high-power transmitters, compromising the survivability of the whole ship.
The reception of weak signals of interest over a wide RF band requires a sensitive and high-linearity receiver. Presence of large unwanted signals makes this task difficult, by reducing the usable spectrum and dynamic range of the receiver. The traditional method of dealing with the interference problem is to use band-stop or notch filters to excise the interferers from the band-of-interest. This approach, employing analog RF components, does not work well when the interferers are numerous, very narrow compared to the passband, and change their spectral locations rapidly. Another approach, called digital-RF, is to digitize the whole band with an analog-to-digital converter (ADC) and perform signal extraction and interference rejection in the digital domain. However, the simple digital-RF approach requires an extremely high dynamic range that is well beyond the current state-of-the-art for high frequencies.
Fixed-tuned filters made from the high temperature superconductors (notably YBCO) have been demonstrated with <0.5 dB insertion loss, 110 dB of rejection with transition slope of 30 dB/100 kHz, and as little as 1% band width. However efforts to develop tuning capability are so far limited to mechanical means and require seconds or more. This is too slow for wide utility. The DARPA FAME program failed to provide a fast, low loss, wide band tuning mechanism. MEMS tuned filters have issues with tuning speed (microsecond tuning at best), and wear out due to mechanical stress after a few billion cycles.
A typical digital-RF receiver front end does not include any tunable analog notch filters for interference rejection. Instead of performing interference rejection, the digital-RF receiver front-end is interference tolerant; it uses an ADC that has sufficient linearity and dynamic range to tolerate the presence of all interferers along with signals-of-interest. Thus, the problem is transferred to the digital domain, where digital filters extract the signals-of-interest. Digital filtering in the frequency domain with an equivalent high order essentially performs the same function as the analog filter that it replaces, albeit with much better flexibility and agility in tuning. Moreover, in the digital domain, one can employ techniques for matched filtering not just in the frequency domain but also in time and/or phase domains for more efficient signal extraction.
In spite of its obvious attraction, the practicality of this digital-RF approach depends on the availability of ADCs with high dynamic range over a wide bandwidth. To get an estimate of the required dynamic range let us consider a single co-site transmitter interfering with a receiver. If the transmitter power is 100 W (50 dBm) and the isolation to the receive antenna is 30 dB, the required dynamic range for receiving a −120 dBm signal-of-interest is 140 dB (˜23 bits). Under certain circumstances, the dynamic range requirement can be as high as 160 dB. Conventional ADCs, based on mature semiconductor technology, cannot achieve this performance over hundreds of MHz of bandwidth (e.g. the 225-400 MHz UHF band, or the 960-1215 MHz L-band).
On the other hand, simple first-order superconductor ADCs have demonstrated spur-free dynamic ranges in the 100 dB range, matching and slightly exceeding the performance of the best semiconductor ADCs. These ADCs employ delta or delta-sigma modulators with extremely high oversampling ratios (the sampling is at 20-40 Gbps, much higher than the bandwidth of ˜200 MHz). Higher sampling rates imply higher oversampling ratio (R) and therefore, larger dynamic range. Since the dynamic range scales as R(2n+1), where n is the order of the modulator, increasing the order of the ADC modulator is expected to improve the dynamic range substantially. Another approach for increasing dynamic range is concatenating the dynamic ranges of multiple ADCs in a subranging architecture.
While the future holds promises of such advances, the large signal interferor persists, and indeed, will persist until the dynamic range of the digitizer exceeds the maximum required dynamic range. Even when such a digitizer becomes available, likely cost and complexity issues will remain.